The present invention relates to monolithic multilayer capacitors such as monolithic multilayer ceramic (MLC) capacitors, and, more particularly, to such a capacitor incorporating a new and highly advantageous thin film end-termination.
A monolithic multilayer capacitor includes a body portion, referred to herein as a "chip," and also includes a pair of end terminations. Such a capacitor has a stacked configuration in contrast to a wound configuration. In the stacked configuration, there are alternating conductive and dielectric layers, with the conductive layers defining first and second sets. These layers are contained in the chip. In the capacitor, all the conductive layers in the first set are electrically connected together at one end of the chip by one of the pair of end terminations; all the conductive layers in the second set are electrically connected together at the opposite end of the chip by the other of the pair of end terminations.
Various types of materials can be used to make a monolithic multilayer capacitor. When ceramic is used as the dielectric, the capacitor is referred to as a monolithic ceramic (MLC) capacitor. Each ceramic layer in an MLC capacitor is very thin, typically only about from 0.0005 inch to 0.003 inch thick. Each conductive layer in an MLC capacitor is also very thin, and is made from a material that has high temperature stability so as to be able to withstand high temperature conditions during processing steps which are carried out in the course of making the laminated or multilayer chip. A platinum/palladium alloy is commonly used for the conductive layers because layers of such material are capable of withstanding such relatively high temperatures.
In use, with voltage applied to the capacitor, an electric field is established between the film electrodes within this thick-film ceramic-electrode layer structure. Occasionally, minor defects in a ceramic dielectric layer allow small leakage currents to flow through the defective dielectric layer, with the result that the temperature of the ceramic rises due to I.sup.2 R heating. Leakage currents tend to increase with increasing temperature, and the whole body becomes progressively hotter. Ultimately, such a runaway situation causes the ceramic layer to break down, and thus MLC capacitors tend to fail by developing electric shorts between adjacent conductive layers.
In recent years, it has become increasingly common to employ many MLC capacitors on a single printed circuit board which also supports a large number of integrated circuit components, for example, MOS memory chips. The MLC capacitors are typically connected directly across a low voltage power supply which has a large current capacity. Such printed circuit boards are typically quite costly. When any one of the many MLC capacitors fails, as described above, its temperature can easily exceed 1,000.degree. C., thus causing ignition and burning of nearby components, and often burning through the supporting board, and possibly causing burning of the boards which are adjacent the board on which the failed MLC capacitor is mounted.
In a typical application, an MLC capacitor functions in the manner of decoupling the power supply from the active components of the electronic circuit, such as integrated circuit (IC) chips, with which the MLC capacitor is used. Typically, this means that the capacitor is connected across the power supply to prevent transmission of any undesirable high frequency components in the power supply output to the integrated circuit components. Thus, with the capacitor connected across the power line, if it fails due to shorting between adjacent electrodes, DC current flows directly through the capacitor, first causing the MLC capacitor to burn, and next causing the IC or other adjacent chip to burn.
The above-described fire-hazard problem does not plague some other types of capacitors such as metallized film capacitors, for example, even though localized defects can occur in any dielectric layer in a metallized film capacitor. Characteristically, metallized film capacitors that have localized defects in a dielectric layer tend to degrade rather than completely fail. This is so because any unduly large leakage current, which flows from one metallized layer through a defect in the adjacent dielectric film to the next adjacent metallized film, burns a hole in at least one of these two metallized layers, thereby breaking the undesired DC conduction path. This opening of the undesired DC conduction path prevents a runaway failure. With other metallized layers, separated by dielectric layers, remaining functional, the capacitor as a whole remains functional, albeit with a lower capacitance.
A second problem encountered with MLC capacitors having defective ceramic layers is that a repetitive but intermittent failure results in a momentary short between electrodes of the MLC capacitor, and then a remission occurs, with the result that a considerable amount of high frequency electric interference is created in the electronic circuitry in which the MLC capacitor is utilized.
Various approaches have been taken in attempts to solve the above-described problems. Elaborate precautions in the form of fire protection systems have been taken in systems such as computer systems containing circuit boards using the MLC capacitors. Further, some designers have utilized different fusible types of elements in series with the capacitor in order to prevent the occurence of the problems which can occur when a capacitor fails short and burns up. Incorporating a fusible element into a capacitor makes the capacitor package both bulky and expensive. In addition, it interferes with its performance and can have the effect of increasing the inductance of the capacitor to the point where, in some cases, the detraction from the performance of the capacitor is such that a fuse can't be used. In addition, fuses have a characteristic which is referred to as equivalent series resistance (ESR). In most instances, the fuses or fusible elements have an ESR which is too high, and the result is that the performance of the capacitor, particularly at high frequencies, is diminished greatly by the inherent ESR of the fusible elements and the extraneous inductance which the element introduces.
Other examples of the manner in which this problem has been addressed are shown in U.S. Pat. Nos. 4,107,759 and 4,193,106. In general, approaches to the problem such as provided by the foregoing two patents all involve additional circuit elements and structural features, including external connections, false terminations, separate fusible wires, soldered or welded connections, or require encapsulation.